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| United States Patent |
5,645,674
|
|
Bohm
, et al.
|
July 8, 1997
|
Methods for achieving improved bond strength between unvulcanized and
vulcanized rubbers
Abstract
An improved method for achieving enhanced bond strength between components
of unvulcanized and at least partially vulcanized rubber the improvement
comprising the steps of selecting first and second initially unvulcanized
rubber components (10, 12) for the manufacture of a vulcanized rubber
article (21); applying an interphase layer (11) of rubber material,
essentially devoid of crosslinking agents and containing from about 0.1 to
about 4 parts by weight of at least one accelerator, per 100 parts by
weight of rubber, to the first component; prevulcanizing the interphase
layer and the first component together until both are at least partially
vulcanized, establishing a gradient crosslink density (43) primarily in
the interphase layer and thereby providing a lower crosslink concentration
at the surface of the interphase layer opposite the first component;
applying the second rubber component to the surface of lower crosslink
density; and covulcanizing the components together, wherein the first and
second unvulcanized rubber components and the interphase layer comprise
rubber selected from the group consisting of natural and synthetic rubber
and blends thereof. A similar improved method for achieving improved bond
strength between components of unvulcanized and at least partially
vulcanized rubbers the improvement comprising the steps of selecting two
unvulcanized rubber components (10, 12) for the manufacture of a
vulcanized rubber article (25, 35); treating one of the components under
conditions that will establish a gradient crosslink density with a lower
crosslink concentration at the surface (43); applying the other rubber
component to the surface; and covulcanizing the components together.
| Inventors:
|
Bohm; Georg G. A. (Akron, OH);
Cetnar; James F. (Uniontown, OH)
|
| Assignee:
|
Bridgestone Corporation (Tokyo, JP)
|
| Appl. No.:
|
344474 |
| Filed:
|
November 23, 1994 |
| Current U.S. Class: |
156/273.5; 156/96; 156/123; 156/130.5; 156/306.9; 156/307.1; 428/493; 428/495; 428/519 |
| Intern'l Class: |
B32B 031/00; B32B 025/04 |
| Field of Search: |
156/96,110.1,123,130.5,273.3,273.5,307.1,307.3,306.9,275.5
428/493-495,515,519
|
References Cited
U.S. Patent Documents
| 1068691 | Jul., 1913 | Moomy | 428/493.
|
| 1274091 | Jul., 1918 | Seward | 156/307.
|
| 1402872 | Jan., 1922 | Langford | 156/307.
|
| 1434892 | Nov., 1922 | Harrison et al. | 156/307.
|
| 1478576 | Dec., 1923 | Morton et al. | 156/307.
|
| 1537865 | May., 1925 | Morton | 156/307.
|
| 1537866 | May., 1925 | Morton | 156/307.
|
| 1569662 | Jan., 1926 | Miller | 156/307.
|
| 1640800 | Aug., 1927 | Peterson | 423/473.
|
| 1777960 | Oct., 1930 | Cadwell | 156/307.
|
| 2206441 | Jul., 1940 | Winkelmann et al. | 156/307.
|
| 2570829 | Oct., 1951 | Maxey et al. | 156/307.
|
| 3372078 | Mar., 1968 | Fausti et al. | 428/519.
|
| 3485712 | Dec., 1969 | Rehm | 161/240.
|
| 4089360 | May., 1978 | Bohm | 152/330.
|
| 4116883 | Sep., 1978 | Seiberling | 428/495.
|
| 4221253 | Sep., 1980 | Seiberling | 152/330.
|
| 4851063 | Jul., 1989 | Seiberling | 156/123.
|
| 5228938 | Jul., 1993 | Kansupada et al. | 156/307.
|
| 5405690 | Apr., 1995 | Hirakawa | 428/327.
|
| Foreign Patent Documents |
| 0 475 388A2 | Sep., 1990 | EP.
| |
| 39 26946A1 | Aug., 1989 | DE.
| |
| 1392218 | Sep., 1972 | GB.
| |
Primary Examiner: Knable; Geoffrey L.
Attorney, Agent or Firm: Hall; Daniel N.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation-in-Part of U.S. Ser. No. 08/029,500,
filed Mar. 11, 1993, now abandoned.
Claims
What is claimed is:
1. An improved method for achieving enhanced bond strength between
components of unvulcanized and at least partially vulcanized rubber the
improvement comprising the steps of:
selecting first and second initially unvulcanized rubber components for the
manufacture of a vulcanized rubber article, both said components
containing crosslinking agents;
applying an interphase layer of rubber material, devoid of crosslinking
agents capable of vulcanizing said interphase layer alone, in any amount
capable of vulcanizing said interphase layer and containing from about 0.1
to about 4 parts by weight of at least one accelerator, per 100 parts by
weight of rubber, to said first unvulcanized component;
prevulcanizing said interphase layer and said first component together
whereby said first component becomes vulcanized and a gradient crosslink
density is established in the interphase layer thereby providing a lower
crosslink concentration at the surface of said interphase layer opposite
said first component;
applying said second unvulcanized rubber component to said surface of lower
crosslink density; and
covulcanizing said components together, wherein said first and second
unvulcanized rubber components and said interphase layer comprise rubber
selected from the group consisting of natural and synthetic rubber and
blends thereof, said interphase layer requiring the migration of a portion
of said crosslinking agents from both said first and second rubber
components to become fully vulcanized.
2. A method, as set forth in claim 1, wherein said first unvulcanized
rubber component is natural rubber and said second unvulcanized rubber
component is selected from the group consisting of synthetic rubbers and
blends thereof.
3. A method, as set forth in claim 1, wherein said first unvulcanized
rubber component is selected from the group consisting of synthetic
rubbers and blends thereof and said second unvulcanized rubber component
is natural rubber.
4. A method, as set forth in claim 1, wherein said interphase layer has a
thickness of from about 0.010 inches up to about 0.080 inches.
Description
TECHNICAL FIELD
The present invention provides a method for improving or achieving good
bond strength between layers or plies of vulcanized rubber or partially
vulcanized rubber and of unvulcanized rubber. One application for the
method is in the manufacture of rubber articles having multiple ply
layers, at least two of which are contiguous, one being vulcanized i.e.,
precured, while the other is unvulcanized.
A number of manufacturing processes, including the fabrication of tires,
require or would benefit from the assembly and subsequent covulcanization
of precured and uncured rubber components. In the retreading of tires for
example, in some instances precured treads are applied to a buffed, used
carcass with the aid of a thin, uncured rubber layer. The radiation
precuring of certain tire components has also become a well-accepted
commercial processing technique which allows precured and uncured rubber
components to be interfaced.
BACKGROUND ART
Tires, conveyor belts and reinforced high pressure hoses are typical of but
a few of the articles wherein cured and uncured components can be
contiguously combined. Generally, the manufacture of these articles
involves the assembly of a plurality of layers of fully compounded rubber
that have been reinforced with carbon black and the like. Tires, as a more
particular example, include components such as beads, sidewalls,
carcasses, treads, and belts. In some instances, it may be desirable to
cure one or more of these individual components off site and prior to
assembly and vulcanization of the tire. An advantage of precuring is to
impart integrity to the rubber based component so that it will resist
distortion during subsequent building and assembly operations which, in
turn, allows more precise alignment of components, greater accuracy during
building and, at the end of assembly, improved tires. Also, because the
bead and tread stock components have varying thicknesses, by subjecting
some such structural components to precure, the cure time of the final
product can be decreased. Moreover, the ability to combine vulcanized and
unvulcanized rubber based components would permit a variety of articles to
be manufactured, such as tires, utilizing one or more "standard"
components or elements to which variable elements e.g., treads, can be
bonded.
Accordingly, it has been desirable to precure or vulcanize certain
components either partially or fully prior to overall assembly to produce
the finished article. Unfortunately, however, the bond interface between
contiguous components, one of which is vulcanized and one of which is
unvulcanized, has not been acceptable. One manner of improving the
adhesion calls for the mechanical buffing of the surface of the vulcanized
rubber component, but this is an extra step and cannot always be employed.
The major hurdle to a broader application of this technique of employing
precured together with uncured components has been the lower adhesion
observed between the precured and the uncured compounds following their
covulcanization. The art has attempted to address the issue of developing
or improving the bonding between contiguous rubber layers but has not
always been successful where the layers are cured and uncured.
U.S. Pat. No. 1,274,091, for example, discloses a composite sheet of
vulcanized rubber comprising sheets of uncured rubber that have been
washed and dried, one of which is broken down by passage through rolls and
contains a vulcanized agent, while the other is neither broken down nor
contains a vulcanizing agent. The two uncured sheets are ultimately
covulcanized.
U.S. Pat. No. 1,402,872 provides a method for uniting masses of
vulcanizable rubber by interposing a layer of rubber without sulfur
therebetween and then covulcanizing the multilayer mass.
U.S. Pat. No. 1,434,892 provides a method of forming a sheet of rubber by
combining one ply containing sulfur with a second ply containing an
accelerator and thereafter covulcanizing the multilayer sheet.
U.S. Pat. No. 1,478,576 is directed toward sheet rubber patch materials for
the repair of inflatable rubber articles. The material comprises a
composite including a rubber layer containing a non-migratory accelerator
and a rubber layer containing sulfur.
Other U.S. patents which teach the covulcanization of uncured rubber sheets
each containing different vulcanizing agents and/or amounts thereof
include U.S. Pat. Nos. 1,537,865, 1,537,866, 1,569,662, 1,777,960, and
2,206,441.
Thus, while others have covulcanized rubber sheets comprising different
vulcanizing agents and amounts, the art has not provided a method for
attaining good adhesion between contiguous rubber articles or components,
one of which is vulcanized and one of which is unvulcanized. More
particularly, the art has not recognized heretofore, the existence of
gradient crosslink densities at the interface between cured and uncured
rubber and hence, has not been able to provide good adhesion therebetween.
The use of irradiation to effect a partial cure of at least a portion of a
tire component, other than the tread, followed by conventional cure of the
tire is disclosed in U.S. Pat. Nos. 4,166,883 and 4,221,253 and 4,851,063,
respective divisionals. The foregoing patents do not suggest the use of an
interphase layer according to the present invention.
DISCLOSURE OF THE INVENTION
It is therefore an object of the present invention to provide a method for
bonding together and covulcanizing sheets of rubber or other rubber-based
components, one of which is at least partially vulcanized and the other is
unvulcanized, and to improve the bond strength therebetween.
It is another object of the present invention to provide a method for
bonding together sheets of rubber or other rubber-based components and to
improve the bond strength therebetween, involving the use of an interphase
layer.
It is yet another object of the present invention to provide a method for
bonding together sheets of rubber or other rubber-based components
involving the use of an interphase layer that is devoid of crosslinking
agents such as sulfur, sulfur donors, peroxides, sulfur-less curatives and
the like.
It is yet another object of the present invention to provide a method for
bonding together sheets of rubber or other rubber-based components, one of
which is at least partially vulcanized and having a gradient crosslink
density and the other is unvulcanized.
It is yet another object of the present invention to provide a method for
bonding together sheets of rubber or other rubber-based components, one of
which is at least partially vulcanized and the other is unvulcanized,
which eliminates the procedure of mechanical buffing of the vulcanized
rubber.
It is yet another object of the present invention to provide a method for
bonding together sheets of rubber or other rubber-based parts involving
the use of at least one chemical cure interfering agent.
It is yet another object of the present invention to provide a method for
bonding together sheets of rubber or other rubber-based parts involving
the use of irradiation to provide a gradient crosslink density.
At least one or more of the foregoing objects, together with the advantages
thereof over known methods, which shall become apparent from the
specification which follows, are accomplished by the invention as
hereinafter described and claimed.
In general, the present invention provides an improved method for achieving
enhanced bond strength between components of unvulcanized and at least
partially vulcanized rubber the improvement comprising the steps of
selecting first and second initially unvulcanized rubber components for
the manufacture of a vulcanized rubber article; applying an interphase
layer of rubber material, essentially devoid of crosslinking agents and
containing from about 0.1 to about 4 parts by weight of at least one
accelerator, per 100 parts by weight of rubber, to the first component;
prevulcanizing the interphase layer and the first component together until
both are at least partially vulcanized, establishing a gradient crosslink
density primarily in the interphase layer and thereby providing a lower
crosslink concentration at the surface of the interphase layer opposite
the first component; applying the second rubber component to the surface
of lower crosslink density; and covulcanizing the components together,
wherein the first and second unvulcanized rubber components and the
interphase layer comprise rubber selected from the group consisting of
natural and synthetic rubber and blends thereof.
The present invention also provides an improved method for achieving
improved bond strength between components of unvulcanized and at least
partially vulcanized rubbers the improvement comprising the steps of
selecting two initially unvulcanized rubber components for the manufacture
of a vulcanized rubber article; treating one of the components under
conditions that will establish a gradient crosslink density with a lower
crosslink concentration at the surface to be adhered to the other
component; applying the other rubber component to the surface; and
covulcanizing the components together, wherein the first and second
unvulcanized rubber components comprise rubber selected from the group
consisting of natural and synthetic rubber and blends thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 to 3 schematically depict the covulcanization of two rubber based
components, one of which is prevulcanized, according to the prior art;
FIG. 4 is a cross-section of one layer of a rubber based component which is
unvulcanized;
FIG. 5 is a cross-section of the component depicted in FIG. 4, and the
application of an interphase layer thereon, according to the present
invention, both being subjected to prevulcanization;
FIG. 6 is a cross-section of the layers depicted in FIG. 5, with the
application of a second unvulcanized layer of rubber based component
against the interphase layer and the final covulcanization thereof;
FIG. 7 is a cross-section of one layer of a rubber based component which is
unvulcanized and the application of an interphase layer thereon, both
being subjected to irradiation curing, according to another method of the
present invention;
FIG. 8 is a cross-section of the layers depicted in FIG. 7, with the
application of a second unvulcanized layer of rubber based component
against the interphase layer and the final covulcanization thereof;
FIG. 9 is a cross-section of one layer of rubber based component and the
application of a chemical cure interfering agent onto a surface thereof,
according to a method of the present invention;
FIG. 10 is a cross-section of the layer depicted in FIG. 9, with the
application of a second unvulcanized layer of rubber based component
against the surface treated layer and the final covulcanization thereof;
and
FIGS. 11 to 13 schematically depict the covulcanization of two rubber-based
components and an interphase layer, as depicted in FIGS. 4 to 6.
PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION
As noted hereinabove, practice of the present invention allows the bonding
of rubbers, of the same or different chemical composition, and of
dissimilar states of vulcanization, in a facile manner to achieve improved
bond strengths. The method is useful in the manufacture of rubber goods
from component plies or the like where it is desirable or necessary to
join one component that is unvulcanized with another that is at least
partially vulcanized.
Vulcanization involves crosslinking of the rubber molecules in the
composition with the concentration of crosslink density governing the
physical properties of the vulcanizate. By partially vulcanized is meant
that the component is partially crosslinked and may be fully crosslinked
to the level desired to achieve complete vulcanization. Partial
vulcanization is useful where green strength is too low and, partial
vulcanization will impart enhanced physical properties, to withstand
subsequent manufacturing operations. Also, thicker rubber based components
such as the bead area or the tread stock for tire manufacture may be
partially vulcanized in order to reduce final cure time of the fully
assembled product. Partial vulcanization is also useful where the final
product can be assembled by selecting a particular rubber based component
from a plurality of different such components which is, in turn, combined
with a "standard" component. In the manufacture of tires, for instance, a
standard carcass could be built and prevulcanized for the assembly of a
variety of tires having different performance properties. By substitution
of a treadstock, different tires could be produced having the same
standard carcass.
In any instance, as the rubber is prevulcanized, a problem that develops
during subsequent covulcanization of the components is that upon
completion of the vulcanization a good bond has not developed at the
interface because the at least partially vulcanized component may become
excessively crosslinked at the interface, in effect, embrittling the
rubber at the interface.
The methods of the present invention solve these problems and provide good
bond strength at the interface of the rubber layers or components. They do
so by imparting a gradient crosslink density in the prevulcanized
component such that the state of cure is lower at the surface that is to
become bonded to the adjacent layer of unvulcanized rubber component in
the ultimate covulcanization of the two components.
In one aspect of the present invention, a thin layer of rubber material,
referred to herein as the interphase layer, is applied between the layers
or components of unvulcanized and at least partially vulcanized rubber.
Initially, the interphase layer is applied to an unvulcanized rubber based
component, both of which are subjected to prevulcanization. In another
aspect of the present invention, the unvulcanized rubber based component
is subjected to an irradiation cure under conditions which produce a
gradient crosslink density. Such means include, for instance, high energy
electrons and are useful because the degree of crosslink density can be
varied through the thickness of the component.
In a third aspect of the present invention, the desired gradient crosslink
density is developed by initially applying a cure interfering agent to a
surface of one of the rubber based components, that is to become at least
partially vulcanized. The agent can be applied to the unvulcanized rubber
component which is then subjected to cure conditions sufficient to impart
at least partial vulcanization. However, at the surface, little curing
takes place while cure progresses within the thickness of the component.
Alternatively, the agent can be applied to a surface of the rubber
component subsequent to prevulcanization. In either instance, when the
second, unvulcanized rubber based component is subsequently brought into
contact with the formerly treated surface of the first component, a good
bond is developed between the two components during covulcanization as a
result of the gradient crosslink density being imparted due to the cure
interfering agents.
Returning to the first aspect of the present invention, the composition of
the interphase layer is preferably substantially identical to the
composition of the first rubber based component, for compatibility. The
composition of the interphase layer should also be compatible with the
composition of the second rubber based component. By compatibility is
meant that the rubber selected to form the interphase layer is one which
will allow the interphase layer to adhere well to the first and second
rubber components. Preferably, it will comprise a blend of the two rubbers
used for the first and second rubber based components.
The interphase layer is preferably compounded without any crosslinking
agents, e.g., sulfur; sulfur donors, peroxides, sulfur-less curatives and
the like. For purposes of the present invention, the term crosslinking
agent is employed in conjunction with compounds conventionally known to
vulcanize rubbers, both natural and synthetic. Typical examples include
sulfur; sulfur donors, such as thiuram disulfides and sulfur chloride;
peroxides; sulfur-less curatives, such as selenium and tellurium;
polysulfide polymers; p-quinone dioxime; dibenzoyl-p-quinone dioxime; the
metallic oxides, such as zinc, lead and magnesium oxide; diisocyantes and
the like. As is also known, such curatives can be employed alone to effect
vulcanization or preferably with accelerators, which are not to be
considered as crosslinking agents for practice of the present invention.
Accelerators are nevertheless, employed in the interphase layer in amounts
ranging from about 0.1 to 4 parts per hundred of rubber (phr). Any of the
conventional rubber accelerators can be employed such as the amines,
thiurams, thiazoles, dithiocarbamates except for sulfur donors such as
tetramethylethylenethiuram disulfide, sulfenamides and guanidines, it
being understood that these accelerators are merely illustrative and that
practice of the present invention is not necessarily limited to any
specific accelerator, the presence thereof being merely optional.
Moreover, while an accelerator may contain sulfur, no elemental sulfur or
other crosslinking agents are employed in the interphase layer.
As will also be appreciated by those skilled in the art, accelerators are
employed in conjunction with sulfur curatives, but not necessarily with
non-sulfur curatives, such as for instance, the peroxides and other
compounds disclosed hereinabove. Accordingly, in lieu of an accelerator,
co-agents may be employed with the curative, one representative example
being the use of unsaturated monomers with peroxide curatives which, upon
activation, copolymerize with the rubber polymer and thereby crosslink it.
Reference herein to the presence of at least one accelerator in the
interphase layer is intended to refer to those interphase layer compounds
that are sulfur curable while for compounds curable by another system, the
term accelerator shall include co-agents, as discussed herein.
Rubbers that may be combined via the present invention include natural
rubber and the synthetic rubbers. Synthetic rubbers are well known and
include the ethylene/propylene copolymers, ethylene/propylene/diene
terpolymers, halogenated rubbers, copolymers of a conjugated diene with at
least one monoolefin, conjugated diene homopolymers and mixtures thereof
with and without natural rubber. Natural/synthetic rubber blends can also
be employed containing between about 5 to 95 parts by weight natural
rubber with the remainder being synthetic rubber.
Examples of suitable halogenated polymers include chloroprene,
(2-chloro-1,3-butadiene or neoprene), chlorosulfonated polyethylene,
chloro- and bromobutyl rubber. Neoprenes are generally categorized as
G-types, W-types and T-types, each being well known to those skilled in
the art.
The copolymers may be derived from conjugated dienes such as 1,3-butadiene,
2-methyl-1,3-butadiene-(isoprene), 2,3-dimethyl-1,3-butadiene,
1,2-pentadiene, 1,3-hexadiene and the like, as well as mixtures of the
foregoing dienes. The preferred conjugated diene is 1,3-butadiene.
Regarding the monoolefinic monomers, these include vinyl aromatic monomers
having from 8 to about 20 carbon atoms such as styrene, alpha-methyl
styrene, vinyl naphthalene, vinyl pyridine and the like and optionally one
or more halogen substituents; alkyl acrylates or methacrylates such as
methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate,
butyl methacrylate and the like; unsaturated nitriles such as
acrylonitrile, methacrylonitrile and the like and vinyl halides such as
vinyl chloride, vinylidene chloride, such as isobutene and the like and
aliphatic monoolefins such as isobutene as well as mixtures of the
foregoing monoolefins. The copolymers may contain up to 50 percent by
weight of the monoolefin based upon the total weight of copolymer. A
preferred copolymer is styrene-butadiene rubber (SBR) a copolymer of a
conjugated diene, especially butadiene, and a vinyl aromatic hydrocarbon,
especially styrene.
The above-described copolymers of conjugated dienes and their method of
preparation are well known in the rubber and polymer arts. Many of the
polymers and copolymers are commercially available. It is to be understood
that practice of the present invention is not to be limited to any
particular rubber included hereinabove or excluded. The rubber should be
useful as a tire component although rubber compositions for other rubber
articles may also be selected.
The compositions of the first and second rubber components will each
contain a sufficient amount of one or more crosslinking agents, as
described hereinabove, to effect vulcanization when subjected to curing at
conventional temperatures and times. One or more accelerators can also be
present in either or both first and second rubber components, again in a
conventional amount for such compound and purposes. As will be explained
next, the crosslinking agents and optional accelerators from the first and
second rubber components also assist in the vulcanization of the
interphase layer.
According to the present invention, it is now possible to combine and bond
components comprising the same rubbers e.g. natural rubber to natural
rubber or SBR to SBR or different rubbers, such as natural rubber to SBR,
either of which can be unvulcanized at the time of final covulcanization.
Where one rubber component is to be at least partially vulcanized prior to
contact with the second rubber based component, according to one aspect of
the present invention, a thin interphase layer is employed. Being
substantially devoid of crosslinking agents, the interphase layer receives
sulfur during prevulcanization by diffusion from the adjacent rubber
component (formerly unvulcanized) and, in certain instances accelerators
where the adjacent rubber component contained an accelerator. In this
manner, a gradient crosslink density is established, primarily in the
interphase layer, as will be discussed in conjunction with the drawing
figures hereinbelow. Subsequently, when the at least partially vulcanized
component is bonded to the other, unvulcanized rubber based component, a
much greater bond strength is developed than where at least partially
vulcanized and unvulcanized rubber based components have been covulcanized
heretofore without an interphase layer.
As previously noted, the interphase layer, devoid of crosslinking agents,
is preferably compounded from the same rubbers as the two rubber based
components. The interphase layer may also comprise a block copolymer
compatible with the two rubber based components or other compatible
polymers. Generally, the composition and thickness of the interphase layer
can be optimized for interposition between the two rubber based
components. Nevertheless, a thickness of at least 0.010 inches is deemed
to be practical, up to about 0.080 inches, although other thicknesses are
not necessarily precluded. More particularly, a thickness of about 0.030
inches up to about 0.045 inches is preferred. Generally, a thinner
interphase layer will be employed where the sulfur diffusion rate is low
while a thicker interphase layer will be employed where the sulfur
diffusion rate is high. Also, a thicker interphase layer will be employed
where the amount of accelerator is low or the activity thereof is low
while a thinner interphase layer will be employed where the amount of
accelerator is high or the activity thereof is high.
A preferred method for practice of the present invention is to prevulcanize
together one unvulcanized rubber based component and the interphase layer
following which the second unvulcanized rubber based component and
interphase layer of the prevulcanized product are brought into contact and
subjected to covulcanization. Prevulcanization of the interphase layer and
first rubber based component is conducted for a time and temperature
sufficient to cause or allow development of physical properties to a
fraction of the optimum reached during complete vulcanization. This
fraction usually ranges from 10 to less than 100 percent of optimum, but
is not limited to this range.
It is to be understood that cure conditions will vary depending upon the
rubber compositions, their thicknesses and degree of vulcanizations, pre
and post cure treatment, and thus, no useful purpose will be served by
specifying a range of such conditions nor is such specification necessary.
This is also true where the preferred method employs irradiation, as
discussed hereinabove.
Where it is desired to employ an interfering agent, e.g., a cure retarder
or cure deactivating agent, the interphase layer is not necessary.
Instead, the cure interfering agent is applied to a surface of the rubber
based component that is to become or has been at least partially
vulcanized. Typical cure retarders are well known to those skilled in the
art including, for instance, salicylic acid, phthalic anhydride, benzoic
acid, N-(cyclohexylthio)phthalimide, N-nitrosodiphenylamine, and others as
are known to those skilled in the art. Typical cure deactivating agents
include mineral acids. One method of surface coating is to form a solution
of the interfering agent in a solvent which can then be applied to the
rubber based component in a suitable manner such as, but not limited to
dipping, spraying, roller coating and the like. It is to be understood
that practice of the present invention is not limited to any particular
cure interfering agent disclosed herein or to the method of application.
Similarly, the amount utilized is not a limitation although, for example,
amounts from about 0.1 to about 10 phr will be effective.
FIGS. 1 to 3 schematically show the concentration profiles of crosslinking
agents and crosslinks formed in two rubber components, one of which is
prevulcanized, and one of which is unvulcanized, and subsequently
covulcanized together according to the prior art. In FIG. 1, component 10
is prevulcanized and component 12 is unvulcanized. Component 12 has a
concentration of accelerator 13 and a concentration of sulfur 14.
Component 10 has been prevulcanized with sulfur and an accelerator and
provides a crosslink density 15. During covulcanization, depicted in FIG.
2, the sulfur and accelerator from component 12 diffuse into component 10,
(area 16) contributing to the further crosslinking thereof. After
covulcanizing, as depicted in FIG. 3, the crosslink density 19 of
component 12 is lower near the interface 17 between the two components. In
contrast, the presence of new crosslinks formed in component 10 during
covulcanization (area 18), adding to the crosslink density 15 formed
during prevulcanization, cause the crosslink density 19 to significantly
increase near the interface of component 10. This region of excessive
vulcanization and embrittlement 20 results in decreased adhesion and
possibly premature failure of the article containing components 10 and 12.
Practice of the three aspects of the present invention is depicted in FIGS.
4-10. In FIG. 4, a layer of unvulcanized rubber based component 10,
depicted schematically, has been selected. In FIG. 5, the component 10 has
applied against it an interphase layer 11. These two layers are first
covulcanized following which the unvulcanized layer 12 is applied. In FIG.
6, the three layers have been covulcanized together to form a product 21.
In FIG. 7, a layer of unvulcanized rubber based component 10 depicted
schematically, also has applied against it an interphase layer 11, both of
which are together first subjected to irradiation via suitable means 22,
to develop a gradient crosslink density following which the second,
unvulcanized rubber based component 12 is applied. The interphase layer 11
is optional and when employed, it may optionally contain one or more
radiation inhibitors such as 2-naphthylamine;
6-phenyl-2,2,4-trimethyl-1,2-dihydroquinone and
N,N'-dioctyl-p-phenylenediamine in an effective amount, known to those
skilled in the art. Such amounts range, for instance, between about 0.1 to
about 10 phr. Where no interphase layer is employed, the gradient
crosslink density occurs by virtue of the irradiation. In FIG. 8, the two
layers have been covulcanized together to form a product 25.
Practice of the third aspect is depicted in FIGS. 9 and 10. A first layer
of unvulcanized rubber based component 10 is treated by means 30 which
applies a coating of cure interfering agent 31 onto a surface 32 of
component 10. The component 10 is then prevulcanized sufficiently to
develop at least a partial vulcanization. The second unvulcanized layer of
rubber component 12 is then applied against the surface 32 and the two
components are covulcanized together, as depicted in FIG. 9 to form the
product 35.
FIGS. 11 to 13 show the concentration profiles of crosslinking agents and
crosslinks formed in rubber components resulting from practice of the
method of the present invention, described in conjunction with FIGS. 4 to
6. FIG. 11 again depicts the two rubber component layers 10 and 12 with an
interphase layer 11 therebetween prior to covulcanization. The interphase
layer 11 is devoid of crosslinking agents, present only in component
layers 10 and 12; however, one or more accelerators are present in the
interphase layer. Component layer 12 has a concentration of accelerator
13, and has been portion of sulfur 14. Component 10 has been prevulcanized
with interphase layer 11 and it will be noted that the crosslink density
thereof 40 decreases at the interface 41 between the two and continues to
decrease across interphase layer 11. Thus, with the diffusion of sulfur
and accelerator during prevulcanization, a gradient crosslink density is
established across layer 11 providing a surface 43 of lower crosslink
concentration.
In FIG. 12, the two components 10 and 12 are being covulcanized and it will
be noted that the sulfur and accelerator from component 12 diffuse into
interphase layer 11, and into component 10 (area 45), contributing to the
further crosslinking thereof. In FIG. 13, it is evident that the
crosslinks formed during prevulcanization 40, together with the crosslinks
generated as a result of the sulfur diffusion occurring during
covulcanization (area 47) result in a more even crosslink distribution 46
throughout the composite and avoid the excessive vulcanization near the
interface region of the prior art (FIG. 3).
In order to demonstrate the efficacy of the present invention, three rubber
compounds were prepared which included a natural rubber compound (NR); a
styrene butadiene rubber compound (SBR); and an interphase layer (IPL)
comprising an SBR rubber and containing one accelerator. The compositions
of these layers are presented in Tables I to III, with all numbers
representing parts per hundred rubber, based upon 100 total parts by
weight of rubber.
TABLE I
______________________________________
NATURAL RUBBER COMPOSITION
STOCK TYPE: NR
______________________________________
Natural Rubber 100.00
Carbon Black 62.00
ZnO 7.50
Stearic Acid 0.50
Adhesion promoter 0.88
Paraphenylenediamine antioxidant
2.00
Antioxidant 1.00
Primary Sulfenamide accelerator
0.50
Secondary Sulfenamide accelerator
0.30
Sulfur blended in oil 6.25
Phthalimide retarder 0.20
TOTAL 181.13
______________________________________
TABLE II
______________________________________
SBR COMPOSITION
STOCK TYPE: SBR
______________________________________
SBR 100.00
Processing oil 27.60
Carbon Black 54.00
ZnO 2.00
Stearic Acid 2.00
Wax 0.75
Polymerized petroleum resin
3.50
Paraphenylenediamine antioxidant
0.95
Sulfur 2.25
Sulfenamide Accelerator
0.60
Sulfur donor 0.60
TOTAL 194.25
______________________________________
TABLE III
______________________________________
INTERPHASE LAYER WITH ACCELERATOR
STOCK TYPE: SBR +
primary accelerator
______________________________________
SBR 100.00
Processing oil 27.60
Carbon Black 54.00
ZnO 2.00
Stearic Acid 2.00
Wax 0.75
Polymerized petroleum resin
3.50
Diamine antioxidant 0.95
Sulfur 0.00
Sulfenamide Accelerator
0.60
TOTAL 191.40
______________________________________
Peel adhesion studies were conducted utilizing the NR and SBR compounds
with and without the IPL interphase layer. Several combinations of
unvulcanized and vulcanized stocks were bonded together in the following
procedure.
For testing purposes the rubber stocks to be adhered together were first
milled to a thickness of 0.050 inch and cut into 6 inch squares. Rubber
backing material reinforced with polyester cords was also prepared to the
same dimensions. The first layer of backing was laid flat with the cords
running horizontally. To this was applied a second layer of backing with
the cords running vertically. One layer of test stock was then applied to
the double backing layer thus completing one-half of the adhesion test
pad. A second similar laminate was prepared for the second test stock
intended for precuring, but it was also covered with a 6 inch square piece
of interphase layer stock. The thickness of the interphase layer was
between 0.010 inch and 0.080 inch and its composition was described
hereinabove. The interphase layer was then covered with a 6 inch square
piece of 0.006 inch polyester film. This film had either a smooth finish
or a rough finish, as noted hereinbelow. (Where the test was for checking
the adhesion between two unvulcanized test stocks, the interphase layer,
polyester film, and prevulcanization step were omitted.) The second
laminate was prevulcanized with the interphase layer in a positive
pressure mold at 10 approximately 278 psi for a time and temperature which
resulted in a prevulcanization ranging between 50 and 100 percent of the
optimum cure as determined by a shear rheometer. After prevulcanization,
the laminate was demolded and the film was removed. Typical
prevulcanization conditions for an SBR/interphase laminate were 11 minutes
at 165.degree. C.
A 2 inch by 6 inch polyester separator film was placed over one end of the
test stock in the first laminate and arranged perpendicular to the cords
in the second backing layer closest to the test stock. The prevulcanized
laminate with the interphase layer was then laid face-to-face on top of
the first laminate, with the cords in the backings closest to the test
stocks running parallel. The assembled pads were then covulcanized in a
positive pressure mold at approximately 278 psi at a temperature and time
that was varied with the thickness of the interphase layer in direct
proportion but always at least five minutes longer than the optimum cure
time for the test stock in the first laminate, as determined by a
rheometer cure curve. Typical covulcanization conditions for an
SBR/interphase/NR laminate were 10 minutes at 165.degree. C. After
demolding, one inch by six inch test strips were die cut from the adhesion
pad in such a way that the cords closest to the test stocks were parallel
to the long axis of the test strip. The separator film was then removed
and the test piece loaded into an Instron machine for 180.degree. peel
testing.
Peel adhesion values (lb/in) were determined for each of the bonded and
covulcanized laminates as follows: The six inch by one inch strips were
generally tested at a clamp speed of two inches per minute. The strength
of the adhesive bond was measured in pounds per inch of sample width
(typically one inch). At the start of the test the strength to initiate
tearing at the adhesive interface reached a maximum and then was quickly
followed by a succession of smaller maxima and minima for the balance of
the test. The first maximum at the start of the test was taken as the Peak
value, and the smaller maxima and minima were averaged mathematically to
arrive at the Plateau value. Two or more strips from each pad were tested,
and the individual Peak and Plateau values were averaged to arrive at the
final Peak/Plateau results.
Measurements were taken at 23.degree. C. and 100.degree. C. and the results
have been reported in Table IV for 12 separate examples as follows,
comprising SBR as the unvulcanized or prevulcanized rubber component
layers and, comprising SBR and natural rubber (NR) as the unvulcanized
rubber component layers. The SBR, N R and interphase layers were prepared
from compositions provided in Tables I to III. Thicknesses were varied
with appropriate adjustments in cure times.
EXAMPLE NO. 1
Two SBR layers, of a composition shown in Table II, were separately
laminated to two polyester cord-reinforced backing layers and then bonded
together and covulcanized at 165.degree. C. for 18 minutes.
EXAMPLE NO. 2
An SBR layer, laminated to two reinforced backing layers on one side and a
smooth, 0.006-inch thick polyester film on the other side, was precured at
165.degree. C. for 11 minutes. The polyester film was then removed and the
freshly exposed rubber surface was bonded to and then covulcanized with a
laminate comprising an unvulcanized SBR layer and two reinforced backing
layers at a temperature of 165.degree. C. for 16 minutes.
EXAMPLE NO. 3
An SBR layer, laminated to two reinforced backing layers on one side and a
rough, 0.006-inch thick polyester film on the other side, was precured at
165.degree. C. for 13 minutes. The polyester film was then removed and the
freshly exposed rubber surface was bonded to and then covulcanized with a
laminate comprising an unvulcanized SBR layer and two reinforced backing
layers at a temperature at 165.degree. C. for 16 minutes.
EXAMPLE NO. 4
An SBR layer, laminated to two reinforced backing layers on one side and a
smooth polyester film on the other side, was precured at 165.degree. C.
for 13 minutes. The polyester film was then removed and the freshly
exposed surface was first buffed and cleaned, and subsequently bonded to
and covulcanized with a laminate comprising an unvulcanized SBR layer and
two reinforced backing layers at a temperature of 165.degree. C. for 16
minutes.
EXAMPLE NO. 5A
An SBR layer, laminated to two reinforced backing layers on one side and an
interphase layer of 0.045-inch thickness on the other side, was
prevulcanized at 165.degree. C. for 13 minutes. The composition of the
interphase layer was that listed in Table III . The polyester sheet, which
covered one side of the interphase layer, was then removed and the freshly
exposed surface was bonded to and then covulcanized with a laminate
comprising an unvulcanized SBR layer and two reinforced backing layers at
a temperature of 165.degree. C. for 16 minutes.
EXAMPLE NO. 5B
An SBR layer, laminated to two reinforced backing layers on one side and an
interphase layer of 0.045-inch thickness on the other side, was
prevulcanized at 165.degree. C. for 13 minutes. The composition of the
interphase layer was that listed in Table III but without any accelerator.
The polyester sheet, which covered one side of the interphase layer, was
then removed and the freshly exposed surface was bonded to and then
covulcanized with a laminate comprising an unvulcanized SBR layer and two
reinforced backing layers at a temperature of 165.degree. C. for 16
minutes.
EXAMPLE NO. 6
An SBR layer of a composition shown in Table II, and a NR layer of a
composition shown in Table I, were separately laminated to two polyester
cord-reinforced backing layers. The two laminates were then bonded
together and covulcanized at a temperature of 165.degree. C. for 11
minutes.
EXAMPLE 7
An SBR layer, laminated to two reinforced backing layers on one side and a
smooth, 0.006-inch thick polyester film on the other side, was precured at
165.degree. C. to 11 minutes. The polyester film was then removed and a
freshly exposed rubber surface was bonded to and then covulcanized with a
laminate comprising an unvulcanized NR layer, of a composition shown in
Table I, and two reinforced backing layers at a temperature of 165.degree.
C. for 10 minutes.
EXAMPLE 8
An SBR layer, laminated to two reinforced backing layers on one side and a
rough, 0.006-inch thick polyester film on the other side, was precured at
165.degree. C. for 13 minutes. The polyester film was then removed and the
freshly exposed rubber surface was bonded to and then covulcanized with a
laminate comprising an unvulcanized NR layer, of a composition shown in
Table I, and two reinforced backing layers at a temperature of 165.degree.
C. for 10 minutes.
EXAMPLE 9
An SBR layer, laminated to two reinforced backing layers on one side and a
smooth polyester film on the other side, was precured at 165.degree. C.
for 11 minutes. The polyester film was then removed and the freshly
exposed surface was first buffed and cleaned, and subsequently bonded to
and covulcanized with a laminate comprising an unvulcanized NR layer, of a
composition shown in Table I, and two reinforced backing layers at a
temperature of 165.degree. C. for 10 minutes.
EXAMPLE 10A
An SBR layer, laminated to two reinforced backing layers on one side and an
interphase layer of 0.045-inch thickness on the other side, was
prevulcanized at 165.degree. C. for 13 minutes. The composition of the
interphase layer was that listed in Table III. The polyester sheet, which
covered one side of the interphase layer, was then removed and the freshly
exposed surface was bonded to and then covulcanized with a laminate
comprising an unvulcanized NR layer, of a composition shown in Table I,
and two reinforced backing layers at a temperature of 165.degree. C. for
11 minutes.
EXAMPLE 10B
An SBR layer, laminated to two reinforced backing layers on one side and an
interphase layer of 0.045-inch thickness on the other side, was
prevulcanized at 165.degree. C. for 13 minutes. The composition of the
interphase layer was that listed in Table III but without any accelerator.
The polyester sheet, which covered one side of the interphase layer, was
then removed and the freshly exposed surface was bonded to and then
covulcanized with a laminate comprising an unvulcanized NR layer, of a
composition shown in Table I, and two reinforced backing layers at a
temperature of 165.degree. C. for 11 minutes.
TABLE IV
__________________________________________________________________________
PEEL ADHESION VALUES - PEAK/PLATEAU
PEEL ADHESION (LBS/INCH)
COMPONENTS IN LAMINATE 23.degree. C.
100.degree. C.
EXAMPLE
COMPONENT 1
COMPONENT 2 PEAK PLATEAU
PEAK PLATEAU
__________________________________________________________________________
1 SBR (Unvulcanized)
SBR (Unvulcanized) 226 >91 C.
111 >58 C.
2 SBR (Unvulcanized)
SBR (Prevulcanized with smooth polyester
198m)
134B 48 44B
3 SBR (Unvulcanized)
SBR (prevulcanized with rough polyester
--lm)
-- 53 46B
4 SBR (Unvulcanized)
SBR (prevulcanized and then buffed)
189 146B -- --
5A SBR (Unvulcanized)
SBR (prevulcanized with interphase layer).sup.a
264 226B 102 96B
5B SBR (Unvulcanized)
SBR (prevulcanized with interphase layer).sup.b
208 190B -- --
6 NR (Unvulcanized)
SBR (unvulcanized) 212 71A 10 10A
7 NR (Unvulcanized)
SBR (prevulcanized with smooth polyester
110m)
22A 9 6A
8 NR (Unvulcanized)
SBR (prevulcanized with rough polyester
150m)
35A 7 7A
9 NR (Unvulcanized)
SBR (prevulcanized and then buffed)
168 >90 C.
-- --
10A NR (Unvulcanized)
SBR (prevulcanized with interphase layer).sup.a
267 162B 153 98B
10B NR (Unvulcanized)
SBR (prevulcanized with interphase layer).sup.b
100 66A -- --
__________________________________________________________________________
a accelerator present in interphase layer, no crosslinking agents
b no accelerator present in interphase layer, no crosslinking agents
A Smooth Interfacial Tear
B Cohesive Tear at Interface
C Tear through test stock to reinforced backing
As is evident from Table IV, Example Nos. 5A and 10A, according to a method
of the present invention resulted in much improved adhesion values over
Example Nos. 2 and 7 without any interphase layers as well as the
techniques involving the use of smooth, rough and buffed surfaces. For
purposes of comparison, Example Nos. 5B and 10B, employed interphase
layers but which did not contain any accelerators and it is readily
apparent that the bond formed between Components 1 and 2 was not as good,
thereby establishing the efficacy of the present invention in the use of
interphase layers without crosslinking agents but with accelerators. The
plateau values for Examples No. 1 and 9 were reported as greater than (>)
the stated numerical value because tearing to the backing occurred rather
than tearing at the bond interface. Hence, the bond strength at the
interface between the stocks was undetermined, but it is thought to be
greater than the strength of the bond between the stock and the reinforced
backing.
Based upon the foregoing disclosure, it should now be apparent that the use
of the methods described herein will carry out the objects set forth
hereinabove. It should also be apparent to those skilled in the art that
the methods of the present invention can be practiced to achieve improved
adhesion between a variety of rubber layers and components utilized in the
manufacture of tires and other articles built from a plurality of plies or
different components. Similarly, the time, temperatures and pressures for
vulcanization can readily be determined by those skilled in the art.
It is, therefore, to be understood that any variations evident fall within
the scope of the claimed invention and thus, the selection of specific
rubber compositions for the unvulcanized rubber based layers or components
as well as the composition and thickness of the interphase layer can be
determined without departing from the spirit of the invention herein
disclosed and described. Moreover, the scope of the invention shall
include all modifications and variations that fall within the scope of the
attached claims.
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